A grain-oriented electrical steel sheet is a soft magnetic material mainly used as an iron core material of a transformer or generator, and has crystal texture in which <001> orientation which is the easy magnetization axis of iron is highly accumulated into the rolling direction of the steel sheet. Such texture is formed through secondary recrystallization of preferentially causing the growth of giant crystal grains in (110)[001] orientation which is called Goss orientation, when secondary recrystallization annealing (final annealing) is performed in the process of manufacturing the grain-oriented electrical steel sheet.
A conventional procedure for manufacturing such a grain-oriented electrical steel sheet is as follows. A slab containing about 4.5 mass % or less Si and an inhibitor component such as MnS, MnSe, and MN is heated to 1300° C. or more to dissolve the inhibitor component. The slab is then hot rolled to obtain a hot rolled sheet. The hot rolled sheet is optionally hot band annealed. The hot rolled sheet is then cold rolled once, or twice or more with intermediate annealing therebetween, to obtain a cold rolled sheet having a final sheet thickness. The cold rolled sheet is then subjected to primary recrystallization annealing in a wet hydrogen atmosphere, thus forming a primary recrystallization annealed sheet that has undergone primary recrystallization and decarburization. After this, an annealing separator having magnesia (MgO) as a main ingredient is applied to the primary recrystallization annealed sheet, and then final annealing is performed at 1200° C. for about 5 h to develop secondary recrystallization and purify the inhibitor component.
A coating is formed on the surface of such a grain-oriented electrical steel sheet to impart insulation property, workability, rust resistance, and the like. The surface coating is typically composed of a base coating mainly made of forsterite and formed during final annealing and a phosphate-based top coating formed on the base coating. These coatings are formed at high temperature and have a low coefficient of thermal (heat) expansion, and so have an effect of reducing iron loss by applying tension to the steel sheet from the difference in coefficient of thermal expansion between the steel sheet and the coating when decreased to ambient temperature.
This effect is greater when the tension is higher. It is therefore desirable to apply as high tension as possible to the steel sheet. High tension also has an effect of reducing sensitivity to external work or stress (degradation in magnetic property, mainly iron loss, caused by compression, degradation in magnetostrictive property, and degradation in noise property when using the steel sheet as an iron core of a transformer). Thus, the formation of the coating that can apply high tension to the steel sheet is important not only for the improvement in iron loss property but also for other purposes.
Various coatings have been conventionally proposed to satisfy such properties. Journal of the Japan Institute of Metals, Vol. 56, No. 12 (1992), pp. 1428-1434 (NPL 1) describes that the use of ceramic such as TiN with a lower coefficient of thermal expansion to obtain higher tension than a forsterite coating or a phosphate coating improves magnetic property significantly.
JP 2984195 B2 (PTL 1) reports that a coating having high tension property can be formed by containing an appropriate amount of TiN in a forsterite coating. To form a coating having higher tension property, a coating with a higher TiN ratio and a method of manufacturing such a coating are needed. As a method of using pure TiN as the base coating of the grain-oriented electrical steel sheet, the use of chemical or physical vapor deposition has been proposed (for example, JP S63-54767 B2 (PTL 2)). However, industrially implementing this requires a very special facility, causing a significant increase in manufacturing cost.